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Air entrainment is the intentional creation of tiny air bubbles in concrete. The bubbles are introduced into the concrete by the addition to the mix of an air entraining agent, a surfactant (surface-active substance, a type of chemical that includes detergents). The air bubbles are created during mixing of the plastic (flowable, not hardened) concrete, and most of them survive to be part of the hardened concrete. The primary purpose of air entrainment is to increase the durability of the hardened concrete, especially in climates subject to freeze-thaw; the secondary purpose is to increase workability of the concrete while in a plastic state.
While hardened concrete appears solid, it is porous, having small capillaries resulting from the evaporation of water beyond that required for the hydration reaction. A water:cement ratio (w/c) of approximately 0.25 (this means 25 parts water for every 100 parts cement) is required for all the cement particles to hydrate. Water beyond that is surplus and is used to make the plastic concrete more workable or flowable. Most concrete has a w/c of 0.45 to 0.60, which means there is substantial excess water that will not react with cement. Eventually the excess water evaporates, leaving little pores in its place. Environmental water can later fill these voids. During freeze-thaw cycles, the water occupying those pores expands and creates stresses which lead to tiny cracks. These cracks allow more water into the concrete and the cracks enlarge. Eventually the concrete spalls - chunks break off. The failure of reinforced concrete is most often due to this cycle, which is accelerated by moisture reaching the reinforcing steel. Steel expands when it rusts, and these forces create even more cracks, letting in more water.
The air bubbles are typically 10 to 500 micrometres in diameter (0.0004 to 0.02 in) and are closely spaced. The air bubble can be compressed a little, and so the bubbles act to reduce or absorb stresses from freezing. Air entraining was introduced in the 1930s and most modern concrete, especially if subjected to freezing temperatures, is air-entrained. The bubbles contribute to workability by acting as a sort of lubricant for all the aggregates and large particles in a concrete mix.
In addition to entrained air, hardened concrete also contains entrapped air. These are larger bubbles, and are typically less evenly distributed than entrained air. Entrapped air is considered to not make a positive contribution to durability and is undesirable though not entirely avoidable.
In hydraulic engineering, air bubble entrainment is defined as the entrapment of air bubbles and pockets that are advected within the turbulent flow. The entrainment of air packets can be localised or continuous along the air–water interface. Examples of localised aeration include air entrainment by plunging water jet and at hydraulic jump. Bubbles are entrained locally at the intersection of the impinging jet with the surrounding waters. The intersecting perimeter is a singularity in terms of both air entrainment and momentum exchange, and the air is entrapped at the discontinuity between the impinging jet flow and the receiving pool of water. Interfacial aeration is defined as the air entrainment process along an air–water interface, usually parallel to the flow direction.
In hydraulic structures, free-surface aeration is commonly observed: i.e., the white waters. The air bubble entrainment may be localised or continuous along an interface (water jets, spillway chutes). Despite recent advances, there are some basic concerns about the extrapolation of laboratory results to large size prototype structures.